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Mon, 26 Sep 2022 14:36:12 +0000 Received: from MN2PR12MB4192.namprd12.prod.outlook.com ([fe80::462:7fe:f04f:d0d5]) by MN2PR12MB4192.namprd12.prod.outlook.com ([fe80::462:7fe:f04f:d0d5%8]) with mapi id 15.20.5654.025; Mon, 26 Sep 2022 14:36:12 +0000 Date: Mon, 26 Sep 2022 11:36:11 -0300 From: Jason Gunthorpe To: Alistair Popple Cc: linux-mm@kvack.org, Andrew Morton , Michael Ellerman , Nicholas Piggin , Felix Kuehling , Alex Deucher , Christian =?utf-8?B?S8O2bmln?= , "Pan, Xinhui" , David Airlie , Daniel Vetter , Ben Skeggs , Karol Herbst , Lyude Paul , Ralph Campbell , "Matthew Wilcox (Oracle)" , Alex Sierra , John Hubbard , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org, nouveau@lists.freedesktop.org, dri-devel@lists.freedesktop.org, Dan Williams Subject: Re: [PATCH 2/7] mm: Free device private pages have zero refcount Message-ID: References: <3d74bb439723c7e46cbe47d1711795308aee4ae3.1664171943.git-series.apopple@nvidia.com> Content-Type: text/plain; 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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=SkpdPRNP; spf=pass (imf21.hostedemail.com: domain of jgg@nvidia.com designates 40.107.212.63 as permitted sender) smtp.mailfrom=jgg@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com; arc=pass ("microsoft.com:s=arcselector9901:i=1") X-Stat-Signature: 6pys6hewnibjsju3mjcsrgb35rsq7mni X-HE-Tag: 1664202974-303602 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Sep 26, 2022 at 04:03:06PM +1000, Alistair Popple wrote: > Since 27674ef6c73f ("mm: remove the extra ZONE_DEVICE struct page > refcount") device private pages have no longer had an extra reference > count when the page is in use. However before handing them back to the > owning device driver we add an extra reference count such that free > pages have a reference count of one. > > This makes it difficult to tell if a page is free or not because both > free and in use pages will have a non-zero refcount. Instead we should > return pages to the drivers page allocator with a zero reference count. > Kernel code can then safely use kernel functions such as > get_page_unless_zero(). > > Signed-off-by: Alistair Popple > --- > arch/powerpc/kvm/book3s_hv_uvmem.c | 1 + > drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 1 + > drivers/gpu/drm/nouveau/nouveau_dmem.c | 1 + > lib/test_hmm.c | 1 + > mm/memremap.c | 5 ----- > mm/page_alloc.c | 6 ++++++ > 6 files changed, 10 insertions(+), 5 deletions(-) I think this is a great idea, but I'm surprised no dax stuff is touched here? Jason